Alimentary Canal:

• The alimentary canal begins with an anterior opening – the mouth, and it opens out posteriorly through the anus.

  • The mouth leads to the buccal cavity or oral cavity.

    • The oral cavity has a number of teeth and a muscular tongue.

  • Each tooth is embedded in a socket of the jaw bone.

    • This type of attachment is called thecodont.

      • The majority of mammals including human beings form two sets of teeth during their life, a set of temporary milk or deciduous teeth replaced by a set of permanent or adult teeth.

        • This type of dentition is called diphyodont.

          • An adult human has 32 permanent teeth which are of four different types (Heterodont dentition), namely, incisors (I), canine (C), premolars (PM), and molars (M).

        • The arrangement of teeth in each half of the upper and lower jaw in the order I, C, PM, and M is represented by a dental formula which in humans is 2123/2123.

      • The hard chewing surface of the teeth, made up of enamel, helps in the mastication of food.

• The tongue is a freely movable muscular organ attached to the floor of the oral cavity by the frenulum.

  • The upper surface of the tongue has small projections called papillae, some of which bear taste buds.

• The oral cavity leads into a short pharynx which serves as a common passage for food and air.

  • The esophagus and the trachea (windpipe) open into the pharynx.

    • A cartilaginous flap called epiglottis prevents the entry of food into the glottis – opening of the windpipe – during swallowing.

    • The esophagus is a thin, long tube that extends posteriorly passing through the neck, thorax, and diaphragm, and leads to a ‘J’ shaped bag-like structure called the stomach.

  • A muscular sphincter (gastro-oesophageal) regulates the opening of the esophagus into the stomach.

• The stomach, located in the upper left portion of the abdominal cavity, has four major parts – a cardiac portion into which the esophagus opens, a fundic region, a body (main central region), and a pyloric portion which opens into the first part of the small intestine.

• The small intestine is distinguishable into three regions, a ‘C’ shaped duodenum, a long coiled middle portion jejunum, and highly coiled ileum.

  • The opening of the stomach into the duodenum is guarded by the pyloric sphincter.

  • The ileum opens into the large intestine.

    • It consists of the caecum, colon, and rectum.

      • The caecum is a small blind sac that hosts some symbiotic microorganisms.

        • A narrow finger-like tubular projection, the vermiform appendix which is a vestigial organ, arises from the caecum.

      • The caecum opens into the colon.

        • The colon is divided into four parts – an ascending, a transverse, descending part, and a sigmoid colon, the descending part opens into the rectum which opens out through the anus.

• The wall of the alimentary canal from the esophagus to the rectum possesses four layers namely serosa, muscularis, submucosa, and mucosa.

  • Serosa is the outermost layer and is made up of a thin mesothelium (epithelium of visceral organs) with some connective tissues.

  • Muscularis is formed by smooth muscles usually arranged into an inner circular and an outer longitudinal layer.

    • An oblique muscle layer may be present in some regions.

  • The submucosal layer is formed of loose connective tissues containing nerves, blood, and lymph vessels.

  • In the duodenum, glands are also present in the sub-mucosa.

  • The innermost layer lining the lumen of the alimentary canal is the mucosa.

    • This layer forms irregular folds (rugae) in the stomach and small finger-like foldings called villi in the small intestine.

    • The cells lining the villi produce numerous microscopic projections called microvilli giving a brush border appearance.

• These modifications increase the surface area enormously.

  • Villi are supplied with a network of capillaries and a large lymph vessel called the lacteal.

  • The mucosal epithelium has goblet cells that secrete mucus that help in lubrication.

    • Mucosa also forms glands in the stomach (gastric glands) and crypts in between the bases of villi in the intestine (crypts of Lieberkuhn).

• All four layers show modifications in different parts of the alimentary canal.

Digestive Glands:

• The digestive glands associated with the alimentary canal include the salivary glands, the liver, and the pancreas.

  • Saliva is mainly produced by three pairs of salivary glands, the parotids (cheek), the submaxillary/sub-mandibular (lower jaw), and the sub-linguals (below the tongue).

    • These glands situated just outside the buccal cavity secrete salivary juice into the buccal cavity.

    • The liver is the largest gland of the body weighing about 1.2 to 1.5 kg in an adult human.

      • It is situated in the abdominal cavity, just below the diaphragm, and has two lobes.

      • The hepatic lobules are the structural and functional units of the liver containing hepatic cells arranged in the form of cords.

        • Each lobule is covered by a thin connective tissue sheath called the Glisson’s capsule.

      • The bile secreted by the hepatic cells passes through the hepatic ducts and is stored and concentrated in a thin muscular sac called the gall bladder.

    • The duct of the gall bladder (cystic duct) along with the hepatic duct from the liver forms the common bile duct.

  • The bile duct and the pancreatic duct open together into the duodenum as the common hepato-pancreatic duct which is guarded by a sphincter called the sphincter of Oddi.

  • The pancreas is a compound (both exocrine and endocrine) elongated organ situated between the limbs of the ‘C’ shaped duodenum.

    • The exocrine portion secretes an alkaline pancreatic juice containing enzymes and the endocrine portion secretes hormones, insulin, and glucagon.

Digestion of Food:

• The process of digestion is accomplished by mechanical and chemical processes.

  • The buccal cavity performs two major functions, mastication of food and facilitation of swallowing.

  • The teeth and the tongue with the help of saliva masticate and mix up the food thoroughly.

    • Mucus in saliva helps in lubricating and adhering the masticated food particles into a bolus.

      • The bolus is then conveyed into the pharynx and then into the esophagus by swallowing or deglutition.

      • The bolus further passes down through the esophagus by successive waves of muscular contractions called peristalsis.

      • The gastro-oesophageal sphincter controls the passage of food into the stomach.

        • The saliva secreted into the oral cavity contains electrolytes and enzymes, salivary amylase, and lysozyme.

        • The chemical process of digestion is initiated in the oral cavity by the hydrolytic action of the carbohydrate-splitting enzyme, salivary amylase.

      • About 30 percent of starch is hydrolyzed here by this enzyme (optimum pH 6.8) into a disaccharide – maltose.

        • Lysozyme present in saliva acts as an antibacterial agent that prevents infections.

• The mucosa of the stomach has gastric glands.

• Gastric glands have three major types of cells namely -

  • Mucus neck cells that secrete mucus

  • Peptic or chief cells which secrete the proenzyme pepsinogen

  • Parietal or oxyntic cells secrete HCl and intrinsic factors (factor essential for the absorption of vitamin B12).

• The stomach stores the food for 4-5 hours.

  • The food mixes thoroughly with the acidic gastric juice of the stomach by the churning movements of its muscular wall and is called the chyme.

    • The proenzyme pepsinogen, on exposure to hydrochloric acid, gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach.

    • Pepsin converts proteins into proteoses and peptones (peptides).

    • The mucus and bicarbonates present in gastric juice play an important role in lubrication and protection of the mucosal epithelium from excoriation by highly concentrated hydrochloric acid.

    • HCl provides the acidic pH (pH 1.8) optimal for pepsins.

  • Rennin is a proteolytic enzyme found in the gastric juice of infants which helps in the digestion of milk proteins.

    • Small amounts of lipases are also secreted by gastric glands.

    • Various types of movements are generated by the muscularis layer of the small intestine.

      • These movements help in a thorough mixing up of the food with various secretions in the intestine and thereby facilitate digestion.

    • The bile, pancreatic juice, and intestinal juice are the secretions released into the small intestine.

      • Pancreatic juice and bile are released through the hepato-pancreatic duct.

        • The pancreatic juice contains inactive enzymes – trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases, and nucleases.

        • Trypsinogen is activated by an enzyme, enterokinase, secreted by the intestinal mucosa into active trypsin, which in turn activates the other enzymes in the pancreatic juice.

      • The bile released into the duodenum contains bile pigments (bilirubin and biliverdin), bile salts, cholesterol, and phospholipids but no enzymes.

      • Bile helps in the emulsification of fats, i.e., breaking down the fats into very small micelles.

      • Bile also activates lipases.

  • The intestinal mucosal epithelium has goblet cells that secrete mucus.

    • The secretions of the brush border cells of the mucosa along with the secretions of the goblet cells constitute the intestinal juice or succus entericus.

      • This juice contains a variety of enzymes like disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc.

  • The mucus along with the bicarbonates from the pancreas protects the intestinal mucosa from acid as well as provides an alkaline medium (pH 7.8) for enzymatic activities.

    • Sub-mucosal glands (Brunner’s glands) also help in this.

• The breakdown of biomacromolecules occurs in the duodenum region of the small intestine.

  • The simple substances thus formed are absorbed in the jejunum and ileum regions of the small intestine.

  • The undigested and unabsorbed substances are passed on to the large intestine.

    • No significant digestive activity occurs in the large intestine.

• The functions of the large intestine are

  • Absorption of some water, minerals, and certain drugs.

  • Secretion of mucus which helps in adhering the waste (undigested) particles together and lubricating it for an easy passage.

    • The undigested, unabsorbed substances called feces enter the caecum of the large intestine through the ileocaecal valve, which prevents the backflow of the fecal matter.

    • It is temporarily stored in the rectum till defecation.

• The activities of the gastrointestinal tract are under neural and hormonal control for proper coordination of different parts.

  • The sight, smell, and/or presence of food in the oral cavity can stimulate the secretion of saliva.

    • Gastric and intestinal secretions are also, similarly, stimulated by neural signals.

    • The muscular activities of different parts of the alimentary canal can also be moderated by neural mechanisms, both local and through CNS.

• Hormonal control of the secretion of digestive juices is carried out by local hormones produced by the gastric and intestinal mucosa.

Absorption of Digested Products:

• Absorption is the process by which the end products of digestion pass through the intestinal mucosa into the blood or lymph.

• It is carried out by passive, active, or facilitated transport mechanisms.

  • Small amounts of monosaccharides like glucose, and amino acids, and some electrolytes like chloride ions are generally absorbed by simple diffusion.

    • The passage of these substances into the blood depends upon the concentration gradients.

• However, some substances like glucose and amino acids are absorbed with the help of carrier proteins.

  • This mechanism is called facilitated transport.

  • The transport of water depends upon the osmotic gradient.

    • Active transport occurs against the concentration gradient and hence requires energy.

  • Various nutrients like amino acids, monosaccharides like glucose, and electrolytes like Na+ are absorbed into the blood by this mechanism.

• Fatty acids and glycerol being insoluble, cannot be absorbed into the blood.

  • They are first incorporated into small droplets called micelles which move into the intestinal mucosa.

    • They are re-formed into very small protein-coated fat globules called the chylomicrons which are transported into the lymph vessels (lacteals) in the villi.

  • These lymph vessels ultimately release the absorbed substances into the bloodstream.

    • Absorption of substances takes place in different parts of the alimentary canal, like the mouth, stomach, small intestine, and large intestine.

• However, maximum absorption occurs in the small intestine.

• The absorbed substances finally reach the tissues which utilize them for their activities.

  • This process is called assimilation.

  • The digestive wastes, solidified into coherent faeces in the rectum initiate a neural reflex causing an urge or desire for its removal.

• The egestion of faeces to the outside through the anal opening (defecation) is a voluntary process and is carried out by a mass peristaltic movement.

Disorders of Digestive System:

• The inflammation of the intestinal tract is the most common ailment due to bacterial or viral infections.

  • The infections are also caused by the parasites of the intestine like tapeworm, roundworm, threadworm, hookworm, pinworm, etc.

Jaundice:

• The liver is affected, and skin and eyes turn yellow due to the deposit of bile pigments.

Vomiting:

• It is the ejection of stomach contents through the mouth.

  • This reflex action is controlled by the vomiting center in the medulla.

  • A feeling of nausea precedes vomiting.

Diarrhea:

• The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as diarrhea.

  • It reduces the absorption of food.

Constipation:

• In constipation, the feces are retained within the colon as the bowel movements occur irregularly.

Indigestion:

• In this condition, the food is not properly digested leading to a feeling of fullness.

  • The causes of indigestion are inadequate enzyme secretion, anxiety, food poisoning, overeating, and spicy food.

Protein-energy Malnutrition:

• Dietary deficiencies in proteins and total food calories are widespread in many underdeveloped countries of South and Southeast Asia, South America, and West and Central Africa.

  • This may affect large sections of the population during drought, famine, and political turmoil.

  • This happened in Bangladesh during the liberation war and in Ethiopia during the severe drought in the mid-eighties.

    • PEM affects infants and children to produce Marasmus and Kwashiorkor.

      • Marasmus is produced by a simultaneous deficiency of proteins and calories.

        • It is found in infants less than a year in age if a mother’s milk is replaced too early by other foods which are poor in both proteins and caloric value.

          • This often happens if the mother has second pregnancy or childbirth when the older infant is still too young.

      • In Marasmus, protein deficiency impairs the growth and replacement of tissue proteins; extreme emaciation of the body and thinning of limbs results, in the skin becoming dry, thin, and wrinkled.

        • Growth rate and body weight decline considerably.

        • Even growth and development of the brain and mental faculties are impaired.

      • Kwashiorkor is produced by protein deficiency unaccompanied by calorie deficiency.

        • It results from the replacement of a mother’s milk with a high-calorie low protein diet in a child more than one year in age.

        • Like marasmus, kwashiorkor shows wasting of muscles, thinning of limbs, failure of growth, and brain development.

• But unlike marasmus, some fat is still left under the skin; moreover, extensive edema and swelling of body parts are seen